The type of fluid dispensing apparatuses to which this invention generally relates provides for the flow of fluid through a body cavity in the apparatus and then out a nozzle orifice which directs the flow of the fluid onto a workpiece, for instance. Control of the flow through the nozzle orifice is typically effected through the movement of a valve member in the nozzle orifice. The valve member ordinarily seats in a valve seat formed in the nozzle orifice to close the nozzle orifice preventing fluid flow out of the body cavity. Movement of the valve member away from the nozzle orifice permits fluid to flow out through the orifice at a rate commensurate with the gap between the valve and valve seat.
It is of course very desirable to be able to accurately and adjustably control flow through the nozzle orifice opening. For instance, changes in the viscosity of material passing through the nozzle orifice will alter the rate of flow of material unless compensated for. Presently, time consuming nozzle changes are often required in order to yield the desired fluid flow.
The problem of flow control has been particularly noted in liquid-dispensing apparatuses for dispensing relatively viscous sticky substances, such as adhesives, hot melts, sealing compounds and the like, such as the liquid-dispensing apparatus which is disclosed in U.S. patent application Serial No. 400,373, filed 7/21/82, now U.S. Pat. No. 4,465,212 which is assigned to applicant's assignee, the disclosure of which is hereby incorporated by reference. This dispensing apparatus, or gun, has a generally cylindrical body with an axial bore therein. A nozzle communicates with one end of the bore, and has a nozzle orifice and a valve seat in the nozzle orifice. A needle valve having an elongated stem has its needle end engageable with the valve seat to thereby control the flow of liquid through the nozzle orifice through movement of the needle valve away from and into engagement with the valve seat. The needle valve is ordinarily biased into a closed position through the use of a compression spring which bears against part of a piston assembly carried on the needle valve. Movement of the needle valve away from the nozzle orifice is effected pneumatically, i.e. by the application of air under pressure to move the piston, to thereby move the needle valve against the bias of the spring. Adjustment of the travel of the needle valve of this liquid dispenser is unavailable except in the form of an adjustment of the compression spring tension, which merely adjusts the pneumatic pressure required to move the needle valve between closed and full open position. Precise flow control is therefore only available in such a dispenser by making nozzle changes, which is a time consuming process, as noted earlier.